Conventionally, an image forming device is known, in which air is inhaled into an image forming device main body by means of a cooling fan; and the inhaled air cools portions to be cooled such as an image forming portion, a power source board and the like that are housed in the device main body (e.g., see a patent document 1). In this device, a right wall portion of the image forming device main body is provided with an air inhaling opening. The air inhaling opening is connected to a duct in which cooling air flows, and the cooling fan is disposed in the duct. The image forming portion is disposed near a left wall portion in the image forming device main body and a downstream-side end portion of the duct is opened near the image forming portion. And, the image forming portion is cooled by an airflow blown out from the downstream-side end portion of the duct. On the other hand, the power source board is disposed below the duct. And, the power source board is cooled by an airflow blown out from a branch opening formed through a portion of the duct.